ExecuteOrDelayUntilScriptLoaded(yourFunction, 'sp.js');
   
        function yourFunction() 
        { 

      var options = { url: 'https://mywebsite.sharepoint.com/MyApp/Pages/Default.aspx', title: 'Policy', width: 900, height: 400,dialogReturnValueCallback: onPopUpCloseCallBack };
                               SP.UI.ModalDialog.showModalDialog(options);

                                                    
         }
                
         function onPopUpCloseCallBack(result, returnValue) 
         {
             if(result== SP.UI.DialogResult.OK)
             {

                  SP.UI.Status.removeAllStatus(true);
                  var sId = SP.UI.Status.addStatus("you clicked ok");
                  SP.UI.Status.setStatusPriColor(sId, 'green');
             }
            else if(result== SP.UI.DialogResult.cancel)
             {
                 SP.UI.Status.removeAllStatus(true);
                 var sId = SP.UI.Status.addStatus("you clicked cancel");
                 SP.UI.Status.setStatusPriColor(sId, 'yellow');
             }
        }

I have used the above code to display a page named Default.aspx in a modal popup but unfortunately it gives me following error

Am I doing something wrong Or its not possible to display a page in modal popup in SharePoint Online?

The same code works perfectly on SharePoint 2013 on premise.

Can anyone help?

Thanks.